Continuous solution polymerization processes generally involve the addition of catalyst to a monomer and solvent mixture. The mixture may be back-mixed giving a uniform polymer in an environment with substantially no concentration gradients. WO 94/00500 (Pannell et al.) describes a solution polymerization using metallocene in a continuous stirred tank reactor, which may be in a series reactor arrangement to make a variety of products.
The heat of the polymerization reaction, called an exotherm, can be absorbed by the reaction mixture. Alternatively, or in addition, the heat of reaction can be removed by a cooling system, such as by external cooling of the walls of the reactor vessel, or by internally arranged heat exchange surfaces cooled by a heat exchange fluid.
In the course of the polymerization, typically a predominant amount of the monomers is consumed and the polymers formed are dissolved in the solvent. Usually, the higher the concentration of the polymer, the higher the viscosity of the polymerization reaction mixture containing the polymer, solvent, and unreacted components. The mixture passes from the polymerization reactor to a finishing section in which polymer, solvent and unreacted monomer are separated. In the finishing step, solvent and unreacted monomers are progressively removed from the reaction mixture until the polymer can be formed into a solid pellet or bale. The separated solvent and monomer can be recycled to the polymerization reactor.
Polymer solutions are known to undergo phase separation at the lower critical solution temperature, with phase separation being encouraged by higher temperatures and/or by lower pressures. Solvent selection also influences the conditions where phase separation occurs.
The phenomenon of phase separation is a consideration in the selection of the polymerization solvent. Appropriate polymerization monomer conversions (especially for volatile monomers), temperatures and pressures should be selected for given polymer/solvent combination conditions to avoid unwanted phase separation inside the reactor. For example, solvents such as hexane may require an elevated pressure in excess of 5 MPa to avoid two-phase conditions for olefin polymerization; solvents such as octane can maintain homogeneous one-phase conditions at lower pressures.
The phenomenon of phase separation can be arranged after the reaction step to separate volatile solvent and unreacted monomers on one hand, and polymers on the other hand. In that case, separation at a temperature well above the lower critical solution temperature is encouraged to allow the polymer to form a concentrated phase. Some earlier articles which explain the general principles include: “A Low-Energy Solvent Separation Method,” by T. G. Gutowski et al., Polymer Engineering and Science, March 1983, Vol. 23, No. 4, pp. 230-237; “Lower Critical Solution Temperature Behavior of Ethylene Propylene Copolymers in Multicomponent Solvents” by C. A. Irani et al., in Journal of Applied Polymer Science (1986), Vol. 31, pp. 1879-1899; “Separating Polymer Solutions with Supercritical Fluids,” by Mark A. McHugh et al., in Macromolecules 1985, Vol. 18, pp. 674-680; “Critical dynamics and phase separation kinetics in dynamically asymmetric binary fluids: New dynamic universality class for polymer mixtures or dynamic crossover?” by Hajime Tanaka is in Journal of Chemical Physics 1 Apr. 1994, 100 (7), pp. 5323-5337; “Short Chain Branching Effect on the Cloud Point Pressures of Ethylene Copolymers in Subcritical and Supercritical Propane” by S. J. Han et al., in Macromolecules 1998, Vol. 31, pp. 2533-2538.
The finishing section may also comprise a devolatilizer, in particular a vacuum devolatilizer in which the molten polymer is exposed to a vacuum while being intensively agitated to draw off volatiles such as solvent and residual monomer in order to reduce the level of volatile component(s) in the final polymer to a desired level.
U.S. Pat. Nos. 6,881,800 and 7,163,989 relate to processes and plants for continuous solution polymerization. Such plants and processes include a pressure source, a polymerization reactor downstream of said pressure source, a pressure let-down device downstream of said polymerization reactor, and a separator downstream of said pressure let-down device, wherein said pressure source is sufficient to provide pressure to said reaction mixture during operation of said process plant to produce a single-phase liquid reaction mixture in said reactor and a two-phase liquid-liquid reaction mixture in said separator in the absence of an additional pressure source between said reactor and said separator.
Many types of catalysts are known for olefin polymerization, including Ziegler-Natta, chromium catalysts and single site catalysts (SSC) or metallocene catalysts. The use of single site catalysts is associated with poor solubility in nonpolar, saturated aliphatic hydrocarbons, which are typically used for homogeneous solution polymerization. As a result, an aromatic catalyst solvent, such as toluene may be used. However, this in turn can complicate solvent separation to prevent toluene build up in the reactor and lead to environmental pollution and added maintenance expenditure.
WO 98/02471 discloses a process for polymerizing ethylene, alpha-olefin and optionally diene monomers. The process comprises the steps of contacting: (1) ethylene; (2) at least one C3-C20 aliphatic alpha-olefin; (3) optionally at least one C4-C20 diene; (4) a catalyst, the catalyst comprising (a) a metallocene complex and (b) an activator; and (5) a solvent. The process can be conducted in single or multiple reactors, and if in multiple reactors, the reactors can be configured in series or parallel. Solvent is removed from the polymer stream in an anhydrous first stage solvent recovery operation such that the solids concentration of the product stream is increased by at least 100 percent. Additional solvent is removed in an anhydrous second stage solvent recovery operation from the product of the first stage solvent recovery operation such that the solids concentration of the product stream is in excess of 65 weight percent.
Polymers prepared by ordinary continuous solution polymerizations are known to is contain a relatively high amount of volatile component(s), which often makes further treatment such as back venting in subsequent extrusion necessary. In case the desired polymers have a relatively low viscosity, the pelletization rate of polymers prepared thereby in the finishing section is still not ideal.
Therefore, there remains a need for improved continuous solution processes and plants which provide one or more of the following advantages: producing polymers containing a trace amount of volatile component(s) with minimal requirement for additional equipment and minimal operating costs across a broad range of operating windows including varying polymerization temperatures; producing a broad spectrum of polymers, particularly polymers of widely varying average molecular weights, molecular weight distributions, and/or comonomer contents; enabling production of polymers having useful molecular weights at high temperatures (for example, above 150° C.); and increasing pelletization rate of polymers in the finishing section.
Additional background information may be found in U.S. Pat. Nos. 3,912,698, 5,599,885, WO 94/00500, WO 92/14766, WO2011/087728, WO2011/087729, WO2011/087730, and WO2011/087731.